This invention relates to high strength ultrafiltration membranes and to a process for producing such membranes. More particularly, this invention relates to high strength cellulosic ultrafiltration membranes made from a microporous polymeric base-resistant substrate and a thin cellulose or cellulose ester polymer ultrafiltration layer.
Microporous and open ultrafiltration membranes include thin sheets and hollow fibers generally formed from polymeric material and having a substantially continuous matrix structure containing open pores or conduits of small size. The pore size range for pores of microporous membranes are generally understood to extend from about 0.05 microns to about 10 microns. Composite ultrafiltration (UF) membranes are UF membranes formed on a pre-existing microporous membrane substrate. The composite membranes have better integrity (higher bubble points) than UF membranes cast from the same polymer solutions onto traditional non-woven backing materials such as a non-woven polyester substrate. For example, U.S. Pat. No. 4,824,568 discloses high bubble point membranes that are composites of polyvinylidene fluoride (PVDF) or polyethersulfone solutions coated onto a 0.22 micron PVDF microporous substrate. The PVDF solutions are based on solvents that also soften a portion of the PVDF substrate. It is presently believed that this solvent bonding is necessary in order to prevent delamination of the composite structure. However, European Patent Application number 93,117 495.7 having publication number 0596411A2 teaches that the use of such a solvent system is undesirable since it can soften the microporous substrate. The use of PVDF also is disadvantageous since PVDF is attacked by common cleaning and sanitizing agents such as 0.5N NaOH. These PVDF based composites, therefore, are not appropriate for use in process streams that foul membranes such as serums, fermentation broths or other protein separation processes which then must be cleaned and sanitized by NaOH.
At the present time, ultrafiltration membranes comprised of cellulose are used in applications where low protein binding and low fouling characteristics are required. Cellulose ultrafiltration membranes are formed by immersion casting of a cellulose acetate polymer solution onto a non-woven fabric substrate formed, for example, from polyethylene or polypropylene. The non-woven substrate has relatively large pores, typically in the order of several hundred microns in effective diameter in comparison to the UF layer formed on it. The UF layer is typically bound to some degree to the substrate by mechanical interlocking of the UF layer and the substrate. The cellulose acetate is then hydrolyzed to cellulose by using a strong base such as 0.5N NaOH.
Alternatively, cellulose can be dissolved in solutions of solvents such as dimethylacetamide (DMAC) or N-methyl pyrrolidone (NMP) with the addition of a salt such as lithium chloride. This cellulose solution can be used to form the composite membrane and subsequently eliminate the need for base hydrolysis.
While these composite membranes are considered to be generally satisfactory, they are not considered to be defect free. A defect is an area of the membrane where a void or rupture in the UF layer will allow passage of particles significantly larger than the retention limit dictated by the UF layer. These defects can result from fibers of the non-woven substrate extending though the UF layer or from gas bubbles retained in the solution from which the cellulose acetate layer is precipitated and coagulated which rupture the UF layer. In addition, defects are caused by the relatively high variability of the non-woven substrate thickness which increases the difficulty of achieving a uniformly thick UF layer. The resultant variable UF layer thickness results in variable permeability and retention performance.
Presently available cellulosic membranes have an undesirably low mechanical strength in that they are easily ruptured when subjected even to low back pressure or when folded even to a minor degree. Delamination of presently available cellulosic membranes is commonly observed at low back pressures of about 3 to 15 psi. Since such membranes can be exposed to some back-pressure during use, resistance to delamination under such back-pressure conditions is desirable.
Accordingly, it would be desirable to provide a cellulosic ultrafiltration membrane which is free of defects, exhibits low protein binding, is stable in high pH solutions, is highly resistant to high back pressure and is mechanically stable even when folded. Such a composite membrane could be highly useful for processing protein-containing solutions under conditions of repeated use. It would also maintain membrane integrity while under conditions of pressure normally encountered during use.